Giant Impact Theory
The giant-impact theory contends that a Mars-sized object impacted with the Earth, causing the ejection of the mantle material that formed the Moon. It postulates two possibilities. The first suggests that initial high speed jetting of material resulted from the mantle-mantle interface. The second suggests that the incoming planetisimal broke up in tangential impact or before full interpenetration, ejecting much additional mass from an effective launch point well outside the Earth’s atmosphere.
This model comports fully with the bulk chemistry data gathered from lunar missions. The energy of the impact would have caused vaporization and eventual dissipation of volatiles. Since this event would have occurred at a time when the Earth’s iron core was forming, the mantle had a reduced iron content and an enriched refractory content. Because material from the Earth became part of the Moon’s components, it also accounts for the similarity in oxygen isotopes found in the lunar crust. There is an additional implication that some of the material from the incoming planetisimal was carried into the lunar components, accounting for some the variation in Moon-Earth abundances.
The eventual fate of the debris generated by an impactor after hitting a planet is governed by the Keplerian trajectory. If the total energy is positive, the debris will escape on a hyperbolic trajectory. The alternative is a closed elliptical orbit, which results in eventual impact on the planet’s surface. The impact theory must account for a mechanism by which enough material to form the Moon is ejected beyond the Roche limit. One of the means postulated for this mechanism suggests that both Earth and the impacting body were differentiated and molten at the time of impact. This would cause the mantle material of both bodies to be largely vaporized. The early motion of the ejected material would not be merely a ballistic trajectory likely to return it to the Earth’s surface, but would be governed by the gas pressure gradients in the expanding silicate vapor. Afterwards, the more refractory silicates would re-condense into particles, forming a thin disk beyond the Roche limit, which would have substantial tidal interactions with the Earth.